The liquid-phase fluorination reaction of halogenated hydrocarbons is one of the means of preparing hydrofluorocarbons (also called HFCs). These compounds are particularly useful as substitutes for chlorofluorocarbons (also called CFCs) or even of hydrochlorofluorocarbons (also denoted by the term HCFCs), the commercialization and utilization of which, especially as refrigeration fluid, has been limited following the Montreal Protocol, because of the risks that the release of these products into the atmosphere harms the stratospheric ozone layer.
HFCs are generally prepared by the action of anhydrous hydrofluoric acid (HF) on a chlorinated hydrocarbon, in the presence of a catalyst, by a substitution reaction in which one or more fluorine atoms replace one or more chlorine atoms. This reaction also produces hydrogen chloride (HCl).
This reaction may be carried out in the gas phase or in the liquid phase. The liquid-phase reaction, which is generally carried out at a temperature between 50 and 150° C., has the advantage of requiring a lower reaction temperature than a gas-phase reaction and consequently has, in general, an improved HFC selectivity.
Patent application U.S. Pat. No. 6,538,165 discloses the use of such a liquid-phase fluorination reaction for the simultaneous preparation of difluoromethane and difluorochloromethane (these compounds also being known as the respective names of HFC-32 and HCFC-22). That document mentions the use of an antimony halide as catalyst and explains the drawback of such a process. This drawback is associated with the corrosion of the material of the reactor by the reaction mixture and especially by the combination of the HF and the antimony halide. Such a drawback, which results in a reduction in the lifetime of the reactor and consumption of the catalyst, is also to be considered within the context of the operational safety of the process, which is an essential aspect in the industrial implementation of an HFC manufacturing process.
Application U.S. Pat. No. 6,538,165 solves this problem by recommending particular temperature and pressure conditions allowing the HF not to be in the liquid state. However, one undesirable consequence of this mode of reaction of the HF is that the desired HFC productivity is appreciably lowered.
The object of international patent application U.S. Pat. No. 5,902,912 is also to limit the corrosion of the reactors used for the liquid-phase fluorination of chlorinated organic compounds. It clearly indicates that the superacid character of the reaction media used has the effect of corroding the materials normally used for manufacturing chemical reactors, including corrosion-resistant materials such as alloys based on nickel and chromium, these being known in the art by the names Inconel® and Hastelloy®. The object of this application is to coat the wall of said reactors with a fluoropolymer. Although this application mentions a number of fluoropolymers that can be envisioned for this purpose, and although it mentions the benefit of preheating the reactants, it does not in any way disclose another means of achieving the heat transfer that is frequently required in a liquid-phase fluorination reaction. It simply suggests the standard use of heat transfer through the walls of the reactor. Now, such a process is not very effective because of the poor heat transmission through a fluoropolymer layer.